A good place to start with a question like this is to consider what you yourself feel that this important text has to say about self-awareness and growth, and why this is important to the central character, Esperanza. It is clear from reading the text that as the story continues, Esperanza is forced to confront and think about a number of situations that either happen to her or to others around her in Mango Street that greatly increase her self-awareness and force her to grow, mature and develop. It is these experiences that help Esperanza to develop her character and become the assured and defiant young woman that rejects the life that other women around her lead and plans to leave Mango Street, making her own life and achieving independence on her own terms. Therefore, a suitable thesis statement you could use might be:

Through what Esperanza observes and experiences in the vignettes that make up The House on Mango Street, her character becomes more self-aware and mature about the nature of her life and the pressures facing her.

Such a thesis statement would allow you to pick three critical events from the story where you feel Esperanza has learnt something or hit a turning point in her character development and talk about them, exploring how precisely she matured. Good luck!